That's something i reply on in forums so often,GTX 560Ti and GTX 560 Ti 448 cores are non-identical GPUs at all,the first one is based on the traditional GF114 GPU,but the 448 core is a GF110 GPU,so the GTX 560 Ti is at all aspects is not able to SLI with that 448 core editon
for SLI you need two identical GPUs,and these don't match !